1. Field of the Invention
This invention relates to signal processing for rounding digital data and more particularly, but not exclusively, to the rounding of digital video data.
2. Description of the Prior Art
It often occurs that a video signal having a gentle slope, as shown in FIG. 17 of the accompanying drawings, is processed in a digital video apparatus as a video material. An example of such a video signal is one which serves as a graded background with a character signal or the like superimposed thereon.
In most cases, a video signal with a gentle slope used for such a purpose is quantized into eight-bit data as shown by a dashed line in FIG. 18 of the accompanying drawings. The video signal can also be quantized into ten-bit data as shown by a solid line in FIG. 18. It is clear from FIG. 18 that the quantization steps of the solid line are less pronounced than those of the dashed line. This is because ten-bit quantization is inherently better able to express gradation (by as much as four times) than that eight-bit quantization.
In view of this, the number of video materials quantized into ten-bit data has been increasing in recent years. In spite of this trend, most of contemporary video apparti for recording digital video signals are eight-bit digital VTRs. This situation is attributable to the fact that there is a limitation on the recording density of the video tape for recording the binary data `1`s and `0`s. This existing situation inevitably requires that the low-order two bits of a ten-bit digital video signal, which have been produced with much effort, be truncated when recording the signal into a digital VTR.
In a known recording technique for recording a ten-bit digital video signal by truncating the low-order two bits, the data undergoes rounding processing, i.e. the ten-bit data is converted into eight-bit one by performing rounding processing thereon. In this technique, random noise is used and a two-bit gradation is generated from the probability of generation of different levels of the random noise. A typical rounding-processing circuit 10 is shown in FIG. 19 of the accompanying drawings.
The circuit shown in FIG. 19 is used For carrying out rounding processing From ten to eight bits. First of all, a ten-bit digital video signal (a) input via a terminal 11 is split into data (b) comprising the high-order eight bits and data (c) comprising the low-order two bits. The low-order two-bit data (c) is then supplied to an adder 13 to be added to two-bit noise data (d) received from a random-noise generator 14.
Words of the two-bit noise data (d) that may be output by the random-noise generator 14 are `00`, `01`, `10` and `11`; each of these words are generated at a probability of 1/4. Data produced by the adder 13 comprises 3 bits including a carry-out bit (CO).
As shown in FIG. 20 of the accompanying drawings, the probability of the carry-out bit (CO) being `1` corresponds to the magnitude of the low-order two-bit data (c) of the input ten-bit digital video signal (a). Accordingly, the carry-out bit (CO) is placed at the same bit position as the least significant bit of the high-order eight-bit data (b) of the input ten-bit digital video signal (a) and added to the high-order eight-bit data (b) by means of an adder 12.
In the case of an eight-bit adder 12, the addition is performed by supplying the high-order eight-bit data (b) to one of the input terminals thereof and all zeros to the other input terminal, the carry-out bit (CO) being red to a carry-input terminal of the adder 12.
In the addition of a high-order eight-bit data word (b) of all ones `11111111` and a carry-out bit (CO) of `1`, the result overflows, outputting a `1` as its carry-out bit. In this case, the addition result (e) comprises nine bits. In order to prevent the rounding-processing circuit 10 from producing a result of nine bits, an overflow processing circuit 15 is used to carry out overflow processing and hence output a digital signal (f) comprising only eight bits.
The processing of the above signals is shown in FIGS. 21A-21D of the accompanying drawings. The operation of digital processing is in reality constrained to using numbers. However, for the sake of description simplicity, signal amplitudes are expressed as analog quantities in FIGS. 21A-21D.
A staircase waveform indicating a grading with ten-bit quantization is shown in FIG. 21A. Data to be added to the input digital video signal is shown in FIG. 21B. A staircase waveform indicating a grading with eight-bit quantization is shown in FIG. 21C. The same data as that shown in FIG. 21B to be added to the eight-bit digital video signal is shown in FIG. 21D.
The data shown in FIG. 21D is added to the signal to give the final output digital video signal (F) obtained from the rounding processing at a terminal 16 shown in FIG. 19. The effect of this rounding processing in which addition is performed is that for each ten-bit gradation, the corresponding truncated eight-bit level is raised to the next higher gradation level with a probability increasing in steps of 25%. FIGS. 21A-21D shows an example of the rounding processing for a staircase waveform starting at a level of 200.
When the output digital video signal (f) is converted into an analog signal and observed using a monitor, (not shown) the gradation of the low-order two bits of the ten-bit video signal can be recognized due to the integration effect of the sense of sight.
In the above rounding processing based on random noise, the screen of the monitor is also subject to noise, resulting in lower quality image. In addition, the Frequency band of the random noise may include a subcarrier frequency of the composite video signal. In such a case, unnecessary loci will appear when a video signal that has undergone the rounding processing is observed by means of a vectorscope. On top of that, there is, among other things, a problem that the eight-bit digital data resulting from the rounding processing cannot be restored to the original ten-bit digital data.